1. Field of the Invention
This invention relates to a developing devices which is used in an electrophotographic or electrostatic-recording image forming apparatus, such as a copier, a printer or the like, for developing an electrostatic latent image.
2. Description of the Related Art
Developing devices having various configurations for developing an electrostatic latent image formed on an image bearing member according to an electrophotographic or electrostatic recording method using a developer, comprising magnetic particles or particles including magnetic particles, have been proposed.
FIG. 9 illustrates such a developing devices In FIG. 9, a developing device 101 includes a developing receptacle 2 for accommodating a developer T configured as described above. Stirring means 7 and 8 are provided within the developing receptacle 2. The developer T is stirred and conveyed by the stirring means 7 and 8 within the developing receptacle 2.
In order to convey the developer T within the developing receptacle 2 toward an image bearing member 11 for the purpose of developing an electrostatic latent image formed on the image bearing member 11, the developing device 101 includes a developing roller 3 which serves as a magnetic-particle bearing member or a developer bearing member. Usually, as shown in FIG. 9, the developing roller 3 comprises a fixed magnet roller 3a, and a developing sleeve 3b mounted around the magnet roller 3a so as to be rotatable in the direction of the arrow, for conveying the developer T toward the image bearing member 11. As shown in FIG. 10, a shaft 3c of the developing sleeve 3b is supported on both side walls 2a of the developing receptacle 2 by bearings 9. In FIG. 10, only one of the side walls 2a is shown.
According to this configuration, the developer T is conveyed from an S3 pole to an N1 pole by the rotation of the developing sleeve 3b, and is formed on the developing sleeve 3b as a developer thin layer by being regulated by a regulating blade 4 supported by a supporting plate 5. An N2 pole is a developing main pole, where the developer T develops the electrostatic latent image formed on the image bearing member 11. Then, the developer T is recollected within the developing receptacle 2 by the rotation of the developing sleeve 3b.
In the developing device 101 having the above-described configuration, the developer T circulated and moved within the developing receptacle 2 may be conveyed toward the bearings 9 along the surface of the developing sleeve 3b and leak from the developing receptacle 2 into the main body of the apparatus via the bearings 9, thereby causing various kinds of problems.
In order to prevent such leakage of the developer T, an attempt has been made, as described in Japanese Patent Laid-Open Application (Kokai) No. 2-262127 (1990), in which magnetic members 106 are disposed so as to face both end portions of the developing sleeve 3b along at least a part of the circumference of the developing sleeve 3b with a predetermined gap with the surface of the developing sleeve 3b. A magnetic circuit is formed by the magnetic members 106 and the magnet roller 3a disposed within the developing sleeve 3b so that magnetic lines of force are concentrated onto surfaces of the magnetic members 6 facing the developing sleeve 3b, to form a magnetic brush comprising the developer T in the gap portion along the magnetic lines of force. The leakage of the developer T from the developing receptacle 2 is prevented by the magnetic brush.
In such a conventional configuration, however, as shown in FIG. 11, a portion A where the magnetic lines of force are lacking, in some cases, depending on the arrangement of magnetic poles of the magnet roller 3a, and therefore the magnetic brush comprising the developer T is not formed. As a result, the leakage of the developer T is not prevented.
Particularly, according to an increase in the circumferential speed of the developing sleeve 3b due to an increase in the speed of the apparatus, and the provision of finer particles of the developer in order to improve the picture quality in recent apparatuses, the above-described conventional configuration does not satisfy a severe requirement for sealability against the developer.